1. Field of the Invention
The invention relates generally to optical scanners, and more particularly, to a system and method for improving the signal to noise ratio of the CCD (charge coupled device) sensor within an optical scanner.
2. Related Art
Optical scanners are used to capture and digitize images. For example, an optical scanner can be used to capture the image of printed matter on a sheet of paper. The digitized image can then be electronically stored and/or processed with character recognition software to produce ASCII text. The typical optical scanner includes a light source, a linear array of photoelectric sensing elements (generally a CCD sensor), an analog amplifier, an analog to digital converter, a controller and a random access memory (RAM).
The CCD sensor includes a large number (e.g., 2000) of photoelectric sensing elements arranged in a linear array. Each photoelectric sensing element will capture light representing a single pixel of the image. The array will capture a line of pixels. By moving the CCD sensor across a document, the entire document can be scanned one line at a time.
The conversion into digital signals of light reflected from or transmitted through the document takes place in essentially three steps. First, each photoelectric sensing element will convert the light which it receives into an electric charge. The magnitude of the charge will depend on the intensity of the light and the exposure time. Second, the charges from each of the photoelectric sensing elements are converted into analog voltages via the analog amplifier. Finally, the analog voltages are digitized by the analog to digital converter for digital image processing and storage in the RAM.
In conventional optical scanners, the CCD sensor is slowly scanned across a document. The photoelectric sensing elements are continuously exposed. After a fixed exposure time, a line of charges (representing a line of pixels of the image) are dumped from the photoelectric sensing elements to one or more analog shift registers. Once the charges are dumped, the photoelectric sensing elements resume generating charges in response to the light to which they are exposed. However, before the next line of charges can be dumped, the analog shift registers must be cleared and the charges stored therein must be processed.
The processing time for the data in the CCD sensor includes the time required to serially shift a line of charges from the analog shift registers, to convert the charges to analog voltages, to digitize the voltages, to perform any desired image processing and to store the digital representation of each pixel in the RAM. Once all pixels or charges of a line have been processed, the charges of the next line can be dumped from the photoelectric sensing elements. Thus, the time required to process all pixels or charges of a line will be equal to the exposure time of the CCD sensor. Conventional optical scanners fix the exposure time equal to this processing time. For example, if it takes one microsecond (1 .mu.sec) to process each charge or pixel, then a 2000 pixel line would require two milliseconds (2 msec) for full processing. Thus, the exposure time for the CCD sensor would be equal to a fixed two milliseconds.
The CCD sensor will generally have a fixed noise level. Thus, to optimize the signal to noise ratio of the scanner, it is desirable to maximize the optical signal received at the CCD sensing element. By maximizing the optical signal, with a fixed noise level, the signal to noise ratio can be maximized. Conventionally, this has been done by calibrating the CCD sensor so that the fixed exposure time yields a maximum charge (without saturating the CCD sensor) from the highest intensity light expected to be received.
This calibration has been done in optical scanners by exposing the CCD sensor to a reference strip of white material and adjusting the intensity of the lamp to achieve maximum charge. However, as the lamp or other components age, the maximum available light may be insufficient to produce maximum charge output from the CCD sensor. As a result, known optical scanners have been unable to maintain maximum signal to noise ratios from the CCD sensor over the life of the scanner.
It is desirable to provide a mechanism for improving the signal to noise ratio of the CCD sensor to overcome the limitations of the prior art.